Dynamic vacuum treatment to produce aluminum alloys

ABSTRACT

Method and apparatus for producing light metal alloys, in particular aluminum alloys, in which the desired alloying elements are first introduced into a vacuum furnace which is then evacuated, whereafter light metal melt is introduced into the furnace chamber by suction in the form of a free-falling metal jet which is thereby subjected to a vacuum treatment for reducing the contents of impurities, such as hydrogen, sodium, oxides and other non-metallic particles, therein. The free-falling metal jet is given such a configuration with respect to composition and flow pattern, such an average length as well as such velocity and direction that the alloying elements are readily dissolved and mixed into the melt in the vacuum furnace, whereby an alloy of desired quality is ready for casting immediately after termination of the vacuum treatment and alloying process.

United States Patent G josteen et a1.

[4 1 July 22, 1975 1 DYNAMIC VACUUM TREATMENT TO PRODUCE ALUMINUM ALLOYS[75] Inventors: Ole Georg Gjosteen; Trygve Olavson Terum; Aksel OlaAarflot, all of Sunndalsora, Norway [73] Assignee: A/S Ardal OG SunndalVerk, Oslo,

Norway [22] Filed: Aug. 12, 1974 [21] Appl. No.: 496,859

Related U.S. Application Data [63] Continuation-in-part of Ser. No.269,086, July 5,

1972, abandoned.

[30] Foreign Application Priority Data May 5, 1972 Norway 1607/72 July16, 1971 Norway 2730/71 52 U.S. Cl. 75/68 R; 75/93 AC; 75/135; 75/ 138[51] Int. Cl. ..C22c1/02 [58] Field of Search 75/68 R, 135, 138, 93 R,75/93 AC, 49, 62, 65

[56] References Cited UNITED STATES PATENTS 1,750,751 3/1930 Geyer75/135 3,116,998 1/1964 Pagonis....

3,125,440 3/1964 Hornak et al. 75/49 3,230,074 l/l966 Roy ct al. 75/493,321,300 5/1967 Worner 75/49 3,356,489 12/1967 Feichtinger 75/93 R3,676,111 7/1972 Wieser et al. 75/68 R 3,728,108 4/1973 Sifferlen et al.75/135 Primary Examiner-L. Dewayne Rutledge Assistant E.taminerM. .l.Andrews Attorney, Agent, or Firm-Wenderoth, Lind & Ponack with respectto composition and flow pattern, such an average length as well as suchvelocity and direction that the alloying elements are readily dissolvedand mixed into the melt in the vacuum furnace, whereby an alloy ofdesired quality is ready for casting immediately after termination ofthe vacuum treatment and alloying process.

8 Claims, 3 Drawing Figures PATENTED JUL 2 2 ms SHEET SHEET PATENTED JUL2 2 1975 DYNAMIC VACUUM TREATMENT TO PRODUCE ALUMINUM ALLOYS This is acontinuation-in-part of application Ser. No. 269,086. filed July 5, 1972now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method for usein producing light metal alloys, in particular aluminum alloys withelements which are commonly known to be alloyable to aluminum, as forinstance Mg, Si, Fe, Mn etc. In this method a melt is vacuum treated andalloyed and at the same time the content impurities therein, such assodium, hydrogen, oxides and other non-metallic particles, areeffectively reduced. At the same time the melt can also be grainrefined. The invention also comprises an apparatus for carrying out themethod.

In recent years there has been in the aluminum market a steadilyincreasing demand for high speed extrudable alloys and rolling alloys,including a number of special alloys having particular requirements, forinstance alloys with qualities such as high brilliancy after anodizing,the latter alloy being among others charac terized by a low content ofiron.

When producing aluminum alloys for casting in the form of quarterfinished products as for instance direct chilled cast rolling ingots,extrusion billets and wire ingots, the actual alloying work compared tothe production of plain aluminum, i.e. aluminum alloyed with only Fe andSi, involves reduced productivity and capacity with melt treatmentmethods being now employed.

Another disadvantage with this alloy production is that the alloyingelements are usually introduced into the melt, which is stirred by handor mechanically, by means of various tools which then contaminate themelt (confer the properties of the various materials or metals to thealuminum melt).

In the aluminum industry there is further a requirement for purifying orrefining the raw metal, depending upon the fields of use for which thesame is intended. In particular it is necessary in many instances toreduce the contents of sodium, finely dispersed oxide and othernon-metallic particles from the aluminum melt at the same time ashydrogen is degased from the melt. In the production of several alloysit is absolutely necessary to carry out such purification.

SUMMARY OF THE INVENTION The main object of this invention is to providea new method with a corresponding apparatus for use on an industrialscale in a practical and effective manner to be able to reduce thecontents of the above mentioned impurities at the same time as the melt,such as aluminum, is alloyed, by any of the known alloys, to the desiredalloy quality, during which time there may also take place a grainrefining process as well.

The method and apparatus of the invention make possible the productionof light metal alloys, for instance aluminum wrought alloys or aluminumcasting alloys, of high quality in a practical manner, The method is inpractice primarily related to a particular utilization of vacuumtechniques in the melt treatment of aluminum. Although the followingdescription mainly will apply to the treatment of aluminum, it is obvious that this invention can also be employed in connection with theproduction of other light metals and their alloys.

From French Pat. No. 918,574 it is known that aluminum melt can bedegassed whereby the melt is drawn or sucked in the form of a jet into avacuum chamber or furnace. The theory has been that a quicker and moreeffective degassing can be obtained than is possible with vacuumtreatment of a stationary melt. This has been explained in that themetal jet or spray has a low static fluid pressure and a large surfacearea com pared with a stationary melt bath in industrial furnaces inwhich the metal surface area is relatively small and in which the staticfluid pressure increases with increasing depth and thereby makesdegassing of the melt difficult and at a certain critical distance fromthe surface completely prevents degassing thereof.

In the principle of the invention these relationships will have apractical effect. It will appear from the description to follow,however, that the degassing effect and the purification effect aregenerally to a large extent determined by the jet flow pattern which isspecific and can assume several different and complex shapes which are afunction of various conditions to be explained more closely below.

The conventional method of vacuum treatment of aluminum melt on anindustrial scale consists in the introduction of molten aluminum chargeby charge into a vacuum furnace which after the introduction of thealuminum melt is evacuated to the desired vacuum and is maintained inthis condition until the charge of molten aluminum has to a desirable orattainable degree been delivered from those substances or impuritieswhich are to be removed. The conventional method, however, hasshort-comings, in the first place because the attainable degree ofpurification does not always correspond to that which is necessary ordesirable, and in the second place because the time of the treatment isso long that the production capacity is comparatively low and theproduction expenses correspondingly high.

In experiments it has been shown that in vacuum jet degassing ofaluminum melt the metal jet in vacuum will have a varying, butcharacteristic shape and composition determined by a number of differentcontrollable factors.

In general the metal jet or spray is composed of two zones: a) a centralzone in which the metal is mainly in the form of metal foam, and b) anouter zone in which the metal is in drop form.

The distribution of the metal volume in these two zones varies with: a)the pressure in the vacuum cham ber, b) the content of gases in themetal, c) the diameter, the length and the shape of the suction tube,and d) the flow velocity in the jet determined by the factors a)pressure in the vacuum chamber and c) diameter length and shape of thesuction tube. Usually more than 90 percent of the metal will be in thecentral zone. The

weight of aluminum melt per unit volume ata) a certain gas content, b) acertain nozzle shape, and c) a pressure of 1 Torr, can have an order ofmagnitude of 0.0006 g/cm at a distance of 100 cm from the suctionopening, i.e. with more than 99 percent of the volume of the centralzone consisting of gas. Under these circumstances the metal jet will bemore like a spray. In vacuum jet degassing of aluminum melt as discussedabove, the degassing is therefore particularly effective compared tovacuum treatment of a stationary melt and also compared to a number ofother known degassing processes for aluminum melt.

As with a static vacuum treatment, it is difficult to imagine that ahomogeneous nucleus formation of bubbles takes place. The nucleusformation of bubbles has a heterogeneous nature, the nucleus formationof the bubbles taking place a) on the walls in the suction opening, b)on the refractory liner in the transfer channel, and c) on non-metallicparticles in the metal jet itself. Thereby there will also be obtained areduction of these impurities by flotation thereof out of the melt. Inaddition to this degassing from the jet itself in the chamber there is asimultaneous degassing from the melt bath.

By suitable arrangement of the suction device it is possible by means ofthe jet to bring about a strong stirring of the melt bath, and as asufficient number of bubble nuclei are generated in the central zone ofthe jet and introduced into the bath, they provide for increased masstransport of impurities to the surface thereof. This jet shape leads toa core in the central zone of more compact consistency, i.e. theporosity of this core is substantially lower than in the remaining partof the central zone. Turbulence in the jet results in the core in thecentral zone also being exposed to vacuum, and both the degassing andthe purifying effect is maintained by giving the metal jet a certainaverage length during the vacuum treatment.

By taking measures such a) that the average length is close to theminimum magnitude for optimum treatment, b) that the jet forms a smallangle to the horizontal plane, and c) that the direction of the jet inthe horizontal plane is adjusted, there is obtained a strong circulationin the melt bath during the vacuum treatment. Experiments have shownthat this circulation of the melt under vacuum increases thepurification effect with respect to the contents of non-metallicparticles, this purification effect being clearly an agglomerationeffect.

An important aspect per se of this invention is the removal of oxidesand other non-metallic particles from the metal melt by means of thevacuum treatment de scribed.

By carrying out the vacuum jet treatment according to the methoddescribed above, it has been shown by experiments that a number ofalloying elements commonly used for aluminum, such as Mg, Fe and Si, canbe alloyed into the melt when these elements in pure form are broughtinto the empty vacuum chamber or furnace before the start of the vacuumtreatment. It is to be understood that these four specifically mentionedalloying elements are exemplary only. Many other elements are known bythose skilled in the art to be used in aluminum alloying, and theinvention extends to all other such commonly known alloying elements.The intense stirring performed by the metal jet increases the rate ofdissolution of the alloying elements at the same time as these elementsduring dissolving thereof will be very thoroughly mixed into the melt.Addition of a grain refiner is made in the same way.

The primary object of the method of the present invention is toeffectively reduce the content of impurities such as hydrogen, sodiumand non-metallic parti cles in the melt at the same time as the melt isalloyed to the desired alloy quality.

For those alloys requiring a low sodium content, for instance AlMg5 (5%Mg), there have been developed methods of introducing a non-reactivegas, for instance argon, through the suction opening and/or through themelt bath itself under vacuum. The gas introduced mainly serves as atransport gas for vaporized sodium.

The above specific utilization of vacuum techniques, in which thealuminum melt is degassed and purified as far as non-metallic impuritiesand sodium are concerned, at the same time as the melt is alloyed and apossible grain refiner is added thereto, has been designated a dynamicvacuum treatment.

More specifically then the method of producing light metal alloys, inparticular aluminum alloys, according to this invention is characterizedin that the desired alloying elements are first introduced into a vacuumfurnace which is then evacuated, whereafter light metal melt isintroduced into the furnace chamber by suction in the form of afree-falling metal jet thereby being subjected to a vacuum treatment forreducing the content of impurities such as hydrogen, sodium, oxides andother non-metallic particles therein, and that the freefalling metal jetis given such a configuration with respect to composition and flowpattern, such an average length as well as such velocity and directionthat the alloying elements are readily dissolved and mixed into the meltin the vacuum furnace, whereby an alloy of desired quality is ready forcasting immediately after termination of the vacuum treatment andalloying process. By the term free-falling it is meant that the metaljet passes into and through the interior of the chamber withoutobstruction, i.e. the metal is not merely poured into the chamber, nordoes it cascade over baffles.

An apparatus for carrying out this new method is according to theinvention characterized that the vacuum chamber is provided with one ormore nozzles forsucking molten metal into the chamber in the form of oneor more jets.

BRIEF DESCRIPTION OF THE DRAWINGS In the following description theinvention shall be explained more in detail with reference to thedrawings in which:

FIG. 1 shows a simplified vertical section of a first apparatus forcarrying out the invention;

FIG. 2 shows also simplified a horizontal section of another embodimentof an apparatus for carrying out the invention; and

FIG. 3 is an enlarged sectional view of the nozzle.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1 there is shown a vacuumfurnace 1A with a vacuum chamber 1 adapted to be evacuated by means ofpump equipment which is not shown. The vacuum furnace 1A furthercomprises means for the heating thereof, the heating means being notshown in the drawing since such means as well as the pump equipment maybe of conventional construction and arrangement. Molten aluminum issupplied to the furnace 1A by means of a transfer channel 3 carryingmolten aluminum metal from for instance holding or mixing furnaces, fromcrucibles or transferring raw electrolytic metal directly to the vacuumfurnace.

The transfer channel 3 feeds the metal to one or more nozzles 2 openinginto the furnace chamber 1 under a) certain angles to the vertical andhorizontal planes, b) a certain level above the bottom of the chamber,c) a certain level above the metal surface corresponding to a maximummetal content in the chamber, in such a way that the metal jet will havea close to optimum average length during the vacuum treatment, withrespect to degassing and reduction of sodium and non-metallic particlesand impurities, at the same time as the composition and flow pattern ofthe jet determined by the inner diameter, length and shape of thesuction nozzles as well as the pressure in the vacuum chamber and thedirection of the jet result in the desired amount of stirring in view ofthe dissolving and alloying of alloying elements introduced into theempty vacuum furnace before the vacuum treatment takes place. Thealloying elements can be in solid form, possibly as pre-alloys.

The structure and arrangement of the apparatus will to some degree berelated to the geometry of the vacuum chamber, as discussed in thedescription of FIG. 2 below.

The method of carrying out vacuum treatment with the apparatus thus fardescribed consists therein that the desired known alloying elements,such as Mg, Fe and Si, in pure metallic solid form or as pre-alloys arecharged into the empty vacuum chamber or furnace. Thereafter aluminummelt is introduced into the transfer channel 3 towards the nozzles 2being at first blocked until the metal level in the transfer channel hasreached a suitable level, preferably a distance H, as shown in FIG. 3,from to cm higher than the upper edge of the inlet opening to thenozzles, whereupon the nozzles are opened and the metal is sucked intothe pre-evacuated chamber 1 in such a way that from each nozzle 2 thereis ejected a jet of molten aluminum metal into the chamber. It ispreferred to keep the level of molten aluminum in the transfer channel 3in front of the nozzles 2 as much as possible constant at the abovediscussed level.

When the jet of aluminum melt is drawn into the evacuated furnacechamber, there will be an almost momentaneous degassing at the same timeas the contents of sodium and non-metallic impurities is stronglyreduced. At the same time the jet has such composition and flow patternand such velocity and direction that the alloying elements, having beenbrought into the vacuum furnace beforehand, are easily dissolved andmixed into the melt so that the desired alloy quality is ready forcasting immediately after termination of the vacuum treatment. Thepossible addition of a grain refiner can take place in the transferchannel 3 in front of the suction opening or nozzles 2 during thedynamic vacuum treatment. The grain refiner can also be added onbeforehand into the vacuum chamber.

Moreover, there can as known per se, be introduced non-reactive gases inthe aluminum melt during the vacuum treatment. Thus, non-reactive gascan be introduced in a way not shown above the melt bath in the chamber1, through the melt bath by means of feed holes 8 as indicated in thedrawing, or through lances,

at the bottom of the furnace 1A, or in the nozzles 2 by means of feedtubes 7 terminating in each of the nozzles.

The non-reactive gas introduced mainly serves as a 5 transport gas forvaporized sodium at the same time as the reduction in the content ofhydrogen and nonmetallic particles is also improved.

When treating strongly contaminated metal there can additionally beprovided a filter unit 6 closely connected to the suction nozzles 2. Thefilter unit 6 is located in the transfer channel 3 outside the vacuumchamber 1 and consists of one or more tubes with an open end closelyconnected to the suction nozzle 2. The filter tubes are manufactured ofrefractory material and have a certain porosity so that the metal bymeans of the reduced pressure in the vacuum chamber, is drawn throughthe tube walls.

Before suction of the melt into the chamber 1 the suction opening ofeach nozzle 2 is kept closed with a seal 9 comprising a sealing materialadapted to melt or burn under the influence of the molten aluminum inthe transfer channel 3 when the melt has attained a desired leveltherein, whereby the nozzle openings are unblocked and suction into thechamber 1 can take place. The sealing material can for instance be anorganic material capable of being ignited and burned under the influenceof the molten aluminum when it has attained the desired level, or thematerial can be a metal, for instance aluminum with a suitable meltingtemperature. The sealing arrangement can also be provided in a simpleway by arranging a sealing material as for instance asbestos or foamrubber sheet as a gasket between the suction inlet opening and a metalplate. When the transfer channel 3 has been filled the vacuum treatmentstarts by moving the metal plate to the side. This arrangement isemployed when the above mentioned filter unit is used in front of thesuction opening.

FIG. 2 shows an advantageous arrangement of nozzles 12 in an elongatedfurnace 11A. In order to obtain a long jet 14 in the chamber 11 thenozzles 12 are located in an end wall of the furnace, which can forinstance have a cylindrical main shape.

As described above, the result of the dynamic vacuum treatment is to alarge degree determined by a number of parameters and the nutualrelationship thereof. The best result has been obtained with parameterswithin the ranges below:

5O Nonle diameter 20-55 mm Average jet length 800l800 mm Nozzle lengthl5-300 mm Nozzle taper 2 Angle of nozzle to the horizontal plane 3-20Temperature 680780C Pressure in the vaccum chamber less than 20 TORR Itwill be understood that the above method and apparatus can be modifiedwithin given ranges and in various ways without departing from the scopeof this invention.

The advantages obtained by using the teaching of this invention can inshort be summed up as follows:

An almost instantaneous degassing and reduction in contents of finelydispersed oxide and other nonmetallic particles in the melt occurs atthe same time as the melt is alloyed to the desired alloy quality.

The following examples are given to additionally illustrate theinvention and refer to the following parameters of the apparatus usedfor the dynamic vacuum treatment:

Nozzle diameter 35 mm Nozzle length 170 mm Nozzle angle to thehorizontal plane Average jet length Vacuum chamber shape ca. 1000 mmHorizontal cylinder with an inner diameter of about 2400 mm Pressure inthe vacuum chamber less than l TORR Capacit of the vacuum cham er 20metric tons.

The examples refer to the treatment of primary or raw metal.

EXAMPLE 1 Alloy quality 50-S (450 metric tons).

Analysis: Fe 0.l90.24%, Mg 0.43O.47% Si 0.480.52%, the balance aluminum.

Reduction: Hydrogen, 45%; Oxygen, 50%.

Non-metallic inclusions: ab. 50%.

Final level:

Hydrogen: 0.12 i 0.02 ml H /l00 g A] N.T.P. (15) Oxygen: 6 i 1.5 ppm 0(18) Yield of alloying elements substantially equal to 100%. Loss of Mgunrecordable by spectrographic analysis. No introduction of non-reactivegas.

Average capacity: Ab. 1 metric ton per min.

EXAMPLE 2 Alloy AlMgS Mg). (100 metric tons). lntroduction ofnon-reactive gas.

Final level:

Hydrogen:0.l2 ml H- /l00 g A] N.T.P. Sodium:3 ppm. Reduction ofnon-metallic particles: Ab. 40% Negligible loss of Mg.

EXAMPLE 3 Metal quality 99.50% A]. (600 metric tons). No introduction ofnon-reactive gas. Final level:

Hydrogen: 0.10 i 0.03 ml HJIOO g Al N.T.P. Oxygen: less than 1.5 ppm 0.Reduction of non-metallic particles: Ab. 45%.

EXAMPLE 4 Alloy quality AlMg2 (2% Mg). (200 metric tons).

lntroduction of non-reactive gas. Final level:

Hydrogen:0.06 ml H g Al N.T.P.

Sodium: less than 3 ppm.

Negligible loss of Mg.

What is claimed is:

l. A method for producing aluminum alloys by means of a melting andmixing process, said method comprising:

introducing desired alloying elements into a vacuum furnace;substantially reducing the pressure in said vacuum furnace, thuscreating a vacuum therein;

thereafter introducing by suction into said vacuum furnace aluminummelt, having undesirable impurities therein, in the form of afree-falling metal jet and passing said free-falling metal jet withoutobstruction through the interior of said vacuum furnace to form in saidvacuum furnace a melt bath formed by said aluminum melt and saidalloying elements, reducing the content of said impurities in saidfree-falling metal jet, and thereafter directing said free-falling metaljet against said melt bath, thereby dissolving and mixing said alloyingelements into said aluminum melt.

2. A method as claimed in claim 1, wherein said aluminum melt isintroduced into said vacuum furnaceby suction through at least onenozzle having a nozzle opening with an upper edge, and said aluminummelt is fed to said at least one nozzle by means of a transfer channel,the level of molten metal in said transfer channel in front of said atleast one nozzle being maintained substantially constant at a level from10 to 30 centimeters higher than said upper edge of said nozzleopenings.

3. A method as claimed in claim 2, further compris-.

ing adding grain refiner additives in said transfer channel in front ofsaid at least one nozzle during the vacuum treatment.

4. A method as claimed in claim 2, further comprising adding anonreactive gas to said melt during the vacuum treatment.

5. A method as claimed in claim 4, wherein said gas is added to saidmelt through said at least one nozzle.

6. A method as claimed in claim 4, wherein said gas is added into themelt bath in said vacuum furnace.

7. A method as claimed in claim 4, wherein said gas is added into saidvacuum furnace above the melt bath therein.

8. A method as claimed in claim 1, further comprising adding grainrefiner additives into the empty-vacuum furnace before the vacuumtreatment starts.

1. A METHOD FOR PRODUCING ALUMINUM ALLOYS BY MEANS OF A MELTING AND MIXING PROCESS, SAID METHOD COMPRISING: INTRODUCING DESIRED ALLOYING ELEMENTS INTO A VACUUM FURNACE, SUBSTANTIALLY REDUCING THE PRESSURE IN SAID VACUUM FURNACE, THUS CREATING A VACUUM THEREIN, THEREAFTER INTRODUCING BY SUCTION INTO SAID VACUUM FURNACE ALUMINUM MELT, HAVING UNDESIRABLE IMPURITIES THEREIN, IN THE FORM OF A FREE-FALLING METAL JET AND PASSING SAID FREEFALLING METAL JET WITHOUT OBSTRUCTION THROUGH THE INTERIOR OF SAID VACUUM FURNACE TO FORM IN SAID VACUUM FURNACE A MELT BATH FORMED BY SAID ALUMINUM MELT AND SAID ALLOYING ELEMENTS, REDUCING THE CONTENT OF SAID IMPURITIES IN SAID FREE-FALLING METAL JET, AND THEREAFTER DIRECTING SAID FREE-FALLING METAL JET AGAINST SAID MELT BATH, THEREBY DISSOLVING AND MIXING SAID ALLOYING ELEMENTS INTO SAID ALUMINUM MELT.
 2. A method as claimed in claim 1, wherein said aluminum melt is introduced into said vacuum furnace by suction through at least one nozzle having a nozzle opening with an upper edge, and said aluminum melt is fed to said at least one nozzle by means of a transfer channel, the level of molten metal in said transfer channel in front of said at least one nozzle being maintained substantially constant at a level from 10 to 30 centimeters higher than said upper edge of said nozzle openings.
 3. A method as claimed in claim 2, further comprising adding grain refiner additives in said transfer channel in front of said at least one nozzle during the vacuum treatment.
 4. A method as claimed in claim 2, further comprising adding a nonreactive gas to said melt during the vacuum treatment.
 5. A method as claimed in claim 4, wherein said gas is added to said melt through said at least one nozzle.
 6. A method as claimed in claim 4, wherein said gas is added into the melt bath in said vacuum furnace.
 7. A method as claimed in claim 4, wherein said gas is added into said vacuum furnace above the melt bath therein.
 8. A method as claimed in claim 1, further comprising adding grain refiner additives into the empty vacuum furnace before the vacuum treatment starts. 